The present invention relates to a light-emitting device comprising a gallium-nitride-group compound-semiconductor, used in optical devices such as a light-emitting diode, a laser diode, etc. More specifically, a semiconductor light-emitting device, in which the efficiency of light emission is maintained high and the color purity is remarkably improved over prior art devices.
Gallium-nitride-group compound-semiconductors have been increasingly used as the semiconductor material for the visible light emitting devices and for electronic devices having high operating temperature. The development has been significant in the field of blue light-emitting diodes.
A basic method of manufacturing the gallium-nitride-group compound-semiconductors is growing a gallium-nitride-group (GaN group) semiconductor film on the surface of an insulating sapphire substrate by means of metal organic CVD. In a practical process of forming a compound-semiconductor layer of GaN group, a substrate is placed in a reaction tube, metal organic compound gases(tri-methyl-gallium, tri-methyl-aluminum, tri-methyl-indium, etc.) are supplied therein as the material gas for the Group III element, and ammonia, hydrazine, etc. are supplied as the material gas for the Group V element, while the substrate is maintained at a high temperature 900xc2x0 C.-1100xc2x0 C., so as to have an n-type layer, a light-emitting layer and a p-type layer grown on the substrate in a stacked layer structure.
As described above, a light-emitting device using the GaN group compound semiconductor is useful as a device for emitting a blue light. However, with regard to technical advancement, the blue light-emitting device is slightly behind when compared with devices emitting red or green light. A reason for the delay in technical advancement is to have been caused by the difficulty of finding an appropriate GaN group compound-semiconductor material. Accordingly, blue light-emitting devices need to show an improvement in the luminance and the color purity, which have been inferior relative to those of the red and green devices.
One effort for improving the luminance, for example, is a blue light-emitting diode having a MIS structure; where, a high resistance i-type GaN group compound-semiconductor layer doped with a p-type impurity is provided on an n-type GaN group compound-semiconductor. In a device having the MIS structure, however, both the luminance and the light-emitting output tend to be insufficient for practical applications.
FIG. 4 is a cross sectional side view of a conventional light-emitting device using GaN group compound-semiconductor.
Referring to FIG. 4, a buffer layer 2 is formed on a sapphire substrate 1, and an n-type clad layer 3, a light-emitting layer 4, a p-type clad layer 5 and a p-type contact layer 6 are formed, in order from the bottom, on the buffer layer 2 by a metal organic CVD method. A p-side electrode 7 is formed on the p-type contact layer 6, while an n-side electrode 8 is formed on an exposed surface of the n-type clad layer 3. The exposed portion of the n-type clad layer 3 is the result of etching-off a part of the p-type clad layer 5 and the light-emitting layer 4.
The GaN group compound-semiconductor light-emitting device in general has a structure comprising a pn junction formed by crystallographically connecting the p-type region and the n-type region of a semiconductor. Namely, a p-type layer of semiconductor for the p-type region and an n-type layer of semiconductor for the n-type region are stacked. By applying a voltage of positive polarity on the p-type layer and a voltage of negative polarity on the n-type layer, a hole is injected from the p-type layer into the n-type layer via the pn junction, and an electron is injected from the n-type layer into the p-type layer. As a result of re-combination of the electron and the hole at the vicinity of the pn junction, a light having an energy identical to the band gap energy of semiconductor in the pn junction is emitted.
Japanese Laid-open Patent Publication No.6-260680, for example, proposes a GaN group compound-semiconductor light-emitting device having a light-emitting layer of n-type InGaN layer doped simultaneously with a p-type impurity, Zn, and an n-type impurity, Si. The Publication discloses that the strength of blue light emission increases as a result of an increase in the number of the donor-acceptor pair light emissions. According to the Publication, the efficiency of light emission and the strength of light emission have been significantly improved as compared with the so-called MIS structured light-emitting devices.
Japanese Laid-open Patent Publication No.846240 discloses a light-emitting device in which a p-type light-emitting layer is formed by doping an acceptor impurity, which is a p-type impurity, and a donor impurity, which is an n-type impurity. According to the Publication, the light-emitting layer may have holes at a high concentration and the quantity of electron injection that reaches deep into the light-emitting layer may be increased; which increases the number of electrons making the re-combination with the holes, leading to an increased luminance.
Furthermore, Japanese Laid-open Patent Publication No.9-186362 proposes a light-emitting device, in which the light-emitting layer is doped with a donor impurity and an acceptor impurity together. The light is emitted as the result of the electron transition between donor impurity level and valence band, conduction band and acceptor level, or conduction band and valence band.
The light-emitting devices disclosed in the above Publications are different from each other in terms of the structure, whether the light-emitting layer has p-type conduction or n-type conduction. Apart from the structural differences, the above disclosed light-emitting devices exhibit an improved luminous intensity as compared with the so-called MIS structured light-emitting devices having a junction of metal-insulation layer-n-type semiconductor layer, in place of a pn junction.
In the light-emitting devices for use in an outdoor display, the sun light, among other things, readily causes interference with the emission of light. Therefore, a further increase in the luminous intensity of the light-emitting devices is required for reproducing a clear image that offers a high recognition capability.
Each of the light-emitting devices disclosed in the above Publications make use of the light emission related to the impurities level, such as the D-A (donor-acceptor) pair light emission in which a p-type impurity, being an acceptor impurity, and a donor impurity emit the light in pairs.
The light emission related to the impurity level, however, generally exhibits a light having a broad spectrum. In addition, the peak wavelength of the above light-emitting device tends to shift toward the short wave side along with an increase in operating current. The broad spectrum affects the purity of color reproduction. When the light-emitting devices are used for a full-color display, range of the color reproduction is narrowed. If the peak wavelength shifts towards a shorter wavelength, the color reproduction is degraded.
As described above, the light emission characteristics of a GaN group compound-semiconductor light-emitting device using the light emitting principle related to the impurity level remain unsatisfactory, especially when it is used in a full-color display application because the color purity can be degraded, in addition to the insufficiency in the luminance.
Assuming the light-emitting layer 4 of an exemplary device of FIG. 4 is an n-type layer, a pn junction is formed by the n-type light-emitting layer 4 and the p-type clad layer 5 stacked on the surface of the n-type light-emitting layer 4. Namely, the re-combination of electrons and holes that contribute to the light emission takes place at the vicinity of a junction between the light-emitting layer 4 and the p-type clad layer 5. Therefore, it is difficult to increase the efficiency of light emission of a light-emitting layer 4 which has been formed of a certain specific semiconductor material selected for obtaining a light of predetermined wavelength. This is a point of limitation with regard to increasing the strength of light emission.
In a case of Japanese Laid-open Patent Publication No.8-46240, where the light-emitting layer 4 is a p-type conduction type, a pn junction is formed in relation to the n-type clad layer 3 having contact with the light-emitting layer 4. Therefore, the re-combination of electrons and holes that contribute to the light emission takes place at the vicinity of a junction between the light-emitting layer 4 and the n-type clad layer 3. This is a point of limitation with regard to increasing the strength of light emission.
As described above, when a light-emitting layer of InGaN, or the like material, is formed in either an n-type or a p-type conduction type, a pn junction formed in relation to the p-type clad layer or the n-type clad layer becomes the light emitting region. As a result, the electron and the hole can not be efficiently re-combined within the light-emitting layer alone whose material has been selected to obtain a light of a predetermined wavelength. This is a problem in obtaining a sufficient light-emitting efficiency at a desired wavelength.
The present invention addresses the above described problems, and presents a GaN group compound-semiconductor light-emitting device having an improved luminous intensity that makes it suitable for use in the full-color outdoor display.
As a result of an extensive study conducted by the inventor on the p-type impurities and the n-type impurities to be doped in a light-emitting layer in an effort to raise the luminous intensity and improve the color purity of emitted light, it was discovered that not only the luminous intensity but also the color purity are remarkably improved by controlling the concentration of the p-type impurity to a certain specific distribution and limiting it within a certain specific concentration range, and forming a pn junction within a light-emitting layer. It was also discovered that if the distribution and the range of the p-type impurity concentration are controlled to a certain specific condition, the influence is negligibly small even if a light-emitting layer is doped with an n-type impurity for a small quantity.
More specifically, the present invention relates to a GaN group compound-semiconductor light-emitting device comprising a p-type layer, a light-emitting layer and an n-type layer of GaN group compound semiconductor stacked in layers by a metal organic CVD method on a substrate, which exhibits an increase in the luminance and the color purity of emitted light by doping the light-emitting layer with a p-type impurity in a mode where the concentration of p-type impurity at a side having contact with the p-type layer gradually decreases towards the other side having contact with the n-type layer.
In accordance with the above described light-emitting device, and the manufacturing method therefor, the pn junction is formed within the light-emitting layer and exhibits an increased efficiency in the light emission. The p-type impurity is not doped to such a high concentration level at which a light emission between the conduction band and the acceptor level becomes dominating; therefore, the light-emitting device uses a light emission due to the electron transition between conduction band and valence band, instead of using the light emission related to the acceptor level.
In a case of a laser diode, a wave guide layer is generally formed between the light-emitting layer and the n-type and p-type clad layers for confining and guiding a light generated from the light-emitting layer within around the vicinity, and help confining the carrier within the light-emitting layer. The layer for guiding the light has a function that is identical to the clad layer, in that it confines the carrier within the light-emitting layer. Accordingly, the present invention includes among the requirements either of the following structures; leaving each of the p-type and n-type clad layers as a single layer or forming a light guiding layer as described above.
Accordingly, it is an object of the present invention to provide a GaN group compound-semiconductor light-emitting device which comprises an n-type layer, a light-emitting layer and a p-type layer of GaN group compound semiconductor stacked on a substrate. The light-emitting layer is doped with a p-type impurity in a mode where the concentration at a side having contact with the p-type layer gradually decreases towards the side having contact with the n-type layer. As a pn junction is formed within the light-emitting layer, electrons and holes are injected into the light-emitting layer at a more efficient manner. The luminous intensity is also increased.
In a variation of the foregoing, the device of the present invention comprises a GaN group compound-semiconductor light-emitting device, wherein the concentration of the p-type impurity doped into the light-emitting layer is within a range, not lower than 1xc3x971016 cmxe2x88x923 and not higher than 5xc3x971018 cmxe2x88x923, at the side having contact with the n-type layer. This makes it easier to form a pn junction within the light-emitting layer, which leads to a higher luminous intensity and color purity.
In another variation, the device of the present invention comprises a GaN group compound-semiconductor light-emitting device, wherein the concentration of the p-type impurity doped into the light-emitting layer is within a range, not lower than 1xc3x971018 cmxe2x88x923 and not higher than 5xc3x971020 cmxe2x88x923, at the side having contact with the p-type layer. This makes it easier to form a pn junction within the light-emitting layer, which leads to a higher luminous intensity and color purity.
In accordance with the foregoing variations of the present invention, besides the advantage of easier forming of a pn junction within the light-emitting layer, the side of light-emitting layer having contact with n-type layer can be maintained as an n-type region even after the p-type impurity is doped at a certain lowered concentration in the region. Furthermore, by raising the concentration of p-type impurity at the side having contact with the p-type layer up-to a certain specific range, the side is maintained as a p-type region. Thus, a pn junction is surely formed within the light-emitting layer. If concentration of the p-type impurity is controlled to fall within the above described range, the light emission due to the electron transition between conduction band and acceptor level is very small. Therefore, a light emission caused by the transition between conduction band and valence band may be used.
In another variation of the device of the present invention, the light-emitting layer is comprised of In x Ga 1xe2x88x92x N (0 less than xc3x97 less than 1). As the light-emitting layer contains no Al, the deterioration of crystalline property of light-emitting layer may be suppressed.
In another variation of the device of the present invention, the p-type impurity is Mg. As it is difficult for the Mg to form a deep impurity level in a GaN group compound-semiconductor, as compared to Zn and other p-type impurities, a light-emitting layer is obtained in which the light emission related to the acceptor level is difficult to occur.
The present invention also relates to a method for manufacturing a GaN group compound-semiconductor light-emitting device comprising the steps of forming, an n-type layer, a light-emitting layer and a p-type layer of GaN group compound-semiconductor on a substrate by a metal organic CVD method. The light-emitting layer is doped with a p-type impurity in a mode where the concentration at a side having contact with the p-type layer gradually decreases toward the side having contact with the n-type layer. A pn junction is formed within the light-emitting layer.
In a variation of the method of the present invention, the doping of p-type impurity is performed by diffusion of a p-type impurity contained in the p-type layer from the p-type layer containing the p-type impurity. With the present manufacturing method, a light-emitting layer having a graded concentration is formed, where the concentration of the impurity decreases from the side having contact with p-type layer towards the n-type layer.
In another variation of the method of the present invention, the p-type impurity is Mg. As it is difficult for the Mg to form a deep impurity level in a GaN group compound-semiconductor, as compared to Zn and other p-type impurities, a light-emitting layer may be obtained in which the light emission related to the acceptor level is difficult to occur.
In another variation, the device of the present invention comprises a GaN group compound-semiconductor light-emitting device comprising an n-type clad layer, a light-emitting layer and a p-type clad layer of GaN group compound-semiconductor formed on a substrate, wherein the light-emitting layer is formed as a substance of stacked layers, namely, an n-type layer provided in the side of the n-type clad layer and a p-type layer provided in the side of the p-type clad layer. As a pn junction is formed within the light-emitting layer, the injection of electrons and holes into the light-emitting layer is expedited, and the electrons and holes are re-combined in the light-emitting layer in a more efficient manner.
In another variation of the device of the present invention, the device comprises a GaN group compound-semiconductor light-emitting device comprising an n-type clad layer, a light-emitting layer and a p-type clad layer of GaN group compound-semiconductor formed on a substrate, wherein the light-emitting layer is formed as a substance of stacked layers, namely, an n-type layer provided in the side of the n-type clad layer, a p-type layer provided in the side of the p-type clad layer and an i-type layer formed between the n-type layer and the p-type layer. As a pn junction is formed within the light-emitting layer, the injection of electrons and holes into the light-emitting layer is expedited, and the electrons and holes are recombined in the light-emitting layer in a more efficient manner.
The invention itself, together with further objects and advantages, can be better understood by reference to the following detailed description and the accompanying drawings.